1. Field of the Invention:
The invention relates to angular position digital encoders having a high degree of resolution.
2. Description of the Prior Art:
Prior art digital angular position encoders such as those used to digitally encode the angular position of a radar antenna, have employed magnetic or optical sensing devices mounted on a stationary base to measure angular position. The basic encoder technique quantizes angular position by using many narrow markings spaced around the perimeter of a disc. The disc is concentrically attached to the shaft whose angular position is being measured so that, as the shaft is rotated to a given position, digitally coded signals are generated which are a function of the shaft position. When attempting to achieve high resolution of the angular position, the limitation of the number of markings that can be placed on the perimeter of the disc led to gearing and backlash problems which increased the encoders' physical size and cost. For example, 2048 bits of resolution can be typically obtained from a magnetic material spaced on one and one-half mil centers on a one inch diameter disc. Where, however, resolution on the order of 2.sup.16 bits is required, the prior art digital shaft encoders become large and expensive.
Prior art digital angle encoders also include devices which measure the angular position of a first shaft in relation to the angle of rotation of a second shaft between a reference position and a first angle position. However the resolution of the angular position detected by this prior art device was limited by the frequency range of a timing signal which was dependent on the number of magnetic impressions, or other identifying marks that could be placed on the perimeter of a disc. Although the frequency range of the timing signal for the device could be increased, this would be at the expense of the compensation for the variations in rotational speed of the drive motor.
Therefore, there was a need for an improved digital angular encoder for use in applications requiring the digital encoding of an angular shaft position to a high degree of resolution.